A simple radiolabelled rheumatoid factor binding assay for the measurement of circulating immune complexes

A simple assay for circulating immune complexes has been developed and the optimal conditions for reaction defined. Partially purified radiolabelled polyclonal or monoclonal rheumatoid factor was incubated with aggregated IgG or soluble immune complexes and the resultant macromolecule precipitated with 3% polyethylene glycol. Monoclonal rheumatoid factor was much the better reagent, allowing the detection of approx. 0.35microgram/ml or more of aggregated IgG. Soluble immune complexes formed in vitro at between x2--x20 antigen excess were detectable over a wide range of molecular size. Clinical studies indicated that the assay is a useful addition to the currently available techniques for measuring circulating immune complexes.